Russian Foreign Minister Sergei Lavrov has accused Syrian rebel groups of seeking to disrupt plans set to gain sustainable peace in the war - torn country.

The rebel groups said on Wednesday it would only take part in peace talks that the United States and Russia are planning if a deadline was set for a settlement that would force President Bashar al - Assad to leave power.

Lavrov said the opposition groups gave the impression it was " doing everything they can to prevent a political process from starting … and achieve military intervention ".

" We consider such approaches unacceptable, " he said.

Russia has repeatedly said that Assad’s exit should not be a precondition for a political solution. " The National Coalition is not prepared to take part in the conference without preconditions.

These conditions are impracticable. And in general, nobody should be throwing ultimatums around, " Lavrov said at a news conference after talks with Cuban Foreign Minister Bruno Rodriguez Parrilla.

The coalition " has no constructive platform ", he said.

" The only thing that unites them is the demand for Bashar al - Assad ' s immediate exit. But everyone, including our Western partners, understands that this position is unrealistic. "

Lavrov said the coalition was " not the sole representative of the Syrian people " and that groups with members inside Syria - including the National Coordination Body, whose representatives have visited Moscow several times - were ready to take part in the proposed peace conference.

" I hope that sensible forces among the Americans and Europeans will clamp down on those who indulge in such unacceptable and aggressive approaches by the National Coalition and who try to cast this coalition as the sole structure with which the Syrian government should talk, " he said.

Lavrov and US Secretary of State John Kerry said on May 7 they would seek to bring Syrian government and opposition envoys together as soon as possible at an international conference aimed at ending the 26 - month - old conflict. No date has been set.